1. Field of the Invention
The present invention relates to a method of manufacturing diphenylmethane dicarbamic acid diester by reacting an N-phenyl carbamic acid ester with a methylating agent in an aqueous acid solution.
2. Description of the Prior Art
Diphenylmethanediisocyanate (to be referred to as an MDI hereinafter) is a useful compound for use as a material for manufacturing high-quality polyurethane resin.
According to a conventional method of manufacturing the MDI on industrial basis, aniline is reacted with formaldehyde to prepare diaminodiphenylmethane, and the resultant diaminodiphenylmethane is reacted with toxic phosgene to prepare the MDI. In the reaction process with phosgene, a large amount of hydrochloric acid is produced and must be properly treated, thus posing a number of problems.
Another conventional method is proposed to prepare MDI. In this method, an N-phenyl carbamic acid ester is reacted wtih a methylating agent to obtain a diphenylmethane dicarbamic acid ester. The resultant ester is thermally decomposed, to prepare the MDI. In the initial condensation process of this method, in addition to the diphenylmethane dicarbamic acid ester as a binuclear compound, further condensed compounds such as trinuclear and tetranuclear compounds are simultaneously obtained. These oligomers are thermally decomposed to obtain so-called crude MDIs, i.e., polymethylene polyphenylisocyanates. However, these crude MDIs have a lower commercial value than that of a pure MDI prepared by thermally decomposing diphenylmethane dicarbamic acid diester. For this reason, demand has arisen for providing a method of preparing a diphenylmethane dicarbamic acid diester wtih high selectivity.
For example, in Japanese Patent Disclosure (Kokai) No. 55-81850, N-phenyl carbamic acid ester is reacted with an aqueous solution of formaldehyde in an aqueous solution of hydrochloric acid. According to this method, even if a reaction is performed for a long period of time, a great amount of nonreacted materials is left and compounds having three or more nuclei are undesirably produced in a total amount of 20% or more. In this reaction system, the reaction progresses while an organic phase is suspended in an aqueous phase. The organic phase contains both phenyl carbamic acid ester as a starting material and diphenylmethane dicarbamic acid ester as a target product. These materials are further condensed to produce compounds having three or more nuclei. In such a reaction system, an aqueous solution of an acid can be easily separated from an organic layer, by layer separation. However, separation of a product, a nonreacted material, and a byproduct must be achieved by distillation.
In Japanese Patent Disclosure (Kokai) No. 55-57550, an organic acid such as trifluoromethane sulfonic acid and an organic solvent such as nitrobenzene are used to prepare a homogeneous reaction system. This reaction system has a high reaction rate, so that the condensation rate of diphenylmethane dicarbamic acid ester is also high. More specifically, diphenyl carbamic acid diester and diphenylmethane dicarbamic acid diester are present in the reaction chamber at an identical phase, so that compounds having three or more nuclei are also produced. In addition, according to this reaction system, an organic acid must be extracted with water and the desired compound must be separated by distillation, thus complicating the separation process. The acid cannot be reused, in practice.